In digital audio systems there is frequently a need to provide a wider dynamic range than can be achieved using the available transmission and/or recording data and error rates. Such increased dynamic range can be provided by the use of techniques whereby the characteristics of the analog to digital converters (ADC's) and digital to analog converters (DAC's) are not fixed but adapt to the program. In one type of adaptive digital audio system, companders of the logarithmic type (often referred to as wide-band companders) have been used to increase dynamic range. Alternatively, digital techniques have also been employed to provide the adapting function, for example by non-linear quantizing or variable scaling systems.
Both analog and digital companding schemes as applied to digital systems potentially suffer from the defect that while dynamic range (the ratio of the maximum to minimum signals which can be accommodated) is increased, the level of quantizing error becomes variable, leading to perceptible modulation of this error (usually considered as noise) by the signal. In analog companders the audible effects of this modulation can be reduced by band-splitting or sliding band techniques whereby the degradation of signal to noise ratio accompanying a particular signal is confined to the same area of the spectrum as the signal, leaving the noise levels in other parts of the spectrum unaffected. By this means the increase in noise is masked. Examples of band-splitting analog companders are given in U.S. Pat. No. 3,846,719, U.S. Pat. No. 3,903,485 and Journal of the Audio Engineering Society, Vol. 15, No. 4, Oct., 1967, pp. 383-388. Analog companders employing sliding band techniques are described in U.S. Pat. No. Re. 28,426, U.S. Pat. No. 3,757,254, U.S. Pat. No. 4,072,914, U.S. Pat. No. 3,934,190 and Japanese patent application No. 55529/71.
In digital companders involving non-linear quantizing or variable scaling, where the program-adaptation takes place in the digital realm, it is usually impractical to eliminate noise modulation by band-splitting or sliding, and designers have been forced to use fixed response shaping networks (pre- and de-emphasis) to reduce the audibility of the noise variation. Such methods operate not by preventing the modulation of noise in one area of the spectrum by signal in another but by altering the spectrum of the noise in the hope that noise in the most audible range of the spectrum (usually high frequencies) will remain inaudible even when it has risen to its highest level as a result of adaptation in response to a signal at a frequency which will not mask this most audible noise. Unfortunately, this is often a vain hope, and pre-emphasized digital companders usually give audible noise modulation on critical musical material.
The permissible response of a shaping network is a compromise between two incompatible requirements. At the output of the DAC, it is desirable to introduce a large loss at the frequencies at which noise or error is most audible; the input of the ADC will then require the inverse network, giving a large gain at these frequencies. However this gain increases the probability of system overload, and hence reduces the effective dynamic range of the system to wide-band signals. In other words, pre- and de-emphasis do not necessarily increase the dynamic range.